This invention relates to renewal of a protective coating on an article, and, more particularly, to renewing a thermal barrier coating (TBC) system by renewing a bond coat beneath the TBC.
Certain components or articles operating in the gas path environment of a gas turbine engine are subjected to significant temperature extremes and degradation by oxidizing and corrosive conditions. It is common practice in the gas turbine engine art to apply a thermal barrier coating (TBC) system to surfaces of such components to protect them from such environment, while also affording the opportunity to improve the efficiency of the engine by enabling increase in operating temperatures.
TBC systems generally are comprised of a metallic environmental inner coating, generally and herein called a bond coat, applied to an article surface, and an insulating ceramic outer layer, generally applied directly over the bond coat. Typical of such TBC outer coatings is one based on zirconia stabilized with yttria, for example about 92 wt. % zirconia stabilized with about 8 wt. % yttria. One preferred method for application or deposition of a TBC coating is by electron beam physical vapor deposition although plasma spray processes are widely used for turbine engine combustor applications. Apparatus is sold commercially for such uses. This general type of TBC system has been reported for some time as evidenced by such U.S. Patents as U.S. Pat. No. 4,055,705--Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,095,003--Weatherly et al. (patented Jun. 13, 1978); U.S. Pat. No. 4,328,285--Siemers et al. (patented May 4, 1982); U.S. Pat. No. 5,216,808--Martus et al. (patented Jun. 8, 1993) and U.S. Pat. No. 5,236,745--Gupta et al. (patented Aug. 17, 1993).
Most frequently used bond coats for gas turbine engine turbine airfoils and combustor components have been classified into two general types. One is an overlay M Al type in which M is at least one element selected from Fe, Ni, and Co, for example MAl, MAlY, MCrAl, and MCrAlY; the other is diffused aluminide coatings. Both of these types have been widely used and reported in connection with the gas turbine art. MCrAlY type coating has been applied by physical vapor deposition, including sputtering, cathodic arc, and electron beam, as well as by plasma spray processes. Coating composition, microstructure and thickness are controlled by processing parameters. Diffused aluminide coatings have been applied by a variety of methods including, as used in the art, pack cementation, above the pack, vapor phase, chemical vapor deposition and slurry coating processes. The thickness and aluminum content of the end product coating has been controlled by varying coating time, coating temperature and aluminum activity of the coating materials and process. The performance of such coatings often is enhanced by incorporating such elements as Pt, Rh, Pd, Cr, Si, Hf, Zr, and/or Y. With either type of bond coat, elements of the bond coat interdiffuse with an article substrate during processing or operation or both yielding a diffusion zone between the bond coat and the underlying article substrate. The diffusion zone is considered to be part of the bond coat and, hence, the TBC system. As used herein, the term bond coat substrate is intended to mean at least a portion of the remaining bond coat and such diffusion zone between the bond coat and the underlying article substrate.
For gas turbine engine applications, the materials and processing methods chosen for the TBC system are selected to provide resistance to spallation of the ceramic outer layer during thermal cycling of the engine as well as resistance to the oxidizing and corrosive environment in the case of a TBC spallation event. During normal engine operation after time, the TBC system, including the bond coat and the ceramic outer layer, will degrade in certain surface areas most subject to strenuous operating conditions. The bond coat has been observed to interdiffuse with an article substrate in such surface areas during operation to the extent that its protective ability has been reduced below an acceptable level, necessitating the removal and reapplication of a protective coating. A current practice in such repair is to remove the entire TBC system including the bond coat, along with its zone of diffusion with the article substrate, and the outer ceramic layer. After any required repair of the article structure, the entire TBC system, including a new bond coat and a new outer ceramic coating, is reapplied. However, that type of TBC system removal, in which the bond coat diffusion zone is removed, will lead to thinning of article walls. Numerous mechanical property data bases have been reported that show a strong correlation of key mechanical properties (including creep rupture strength and high cycle fatigue capability) and remaining wall thickness. Therefore, such wall thinning can result in reduction in operating life and subsequent repairability of the article, as well as airflow control problems if air cooling openings are involved.